PRODUCT SPECIFICATION

ML4425

REV. 1.0.2 7/2/01

11

Figure 8. ML4425 Start-up Circuitry for Controlling the Align and Ramp Times

CAT

CAT

FB A

FB B

FB C

BACK

EMF

SAMPLER

CRT

CRT

CRR

–

+

1.5V

750nA

–

+

1.5V

750nA

VDD

VDD

CRR

500nA

VDD

SPEED

FB

TO

SPEED FB

FILTER

CVCO

RVCO

VOLTAGE

CONTROLLED

OSCILLATOR

VCO/TACH

TO RESET INPUT

OF COMMUTATION

STATE MACHINE

Run Mode (Back EMF Sensing)

At the end of ramp mode the controller goes into run mode.

In run mode, the back EMF sensing is enabled and commuta-

tion is now under the control of the phase locked loop. Motor

speed is now regulated by the speed control loop.

PWM Speed Control

Speed control is accomplished by setting a speed command

The accuracy of the speed command is determined by the

methods that can be used to control the speed command of

the ML4425. One is to use a 10k

to ground with the wiper connected to SPEED SET. If

SPEED SET is controlled from a microcontroller, one of its

The speed command is compared with the sensed speed from

SPEED FB through a transconductance error amplifier. The

output of the speed error amplifier is SPEED COMP. SPEED

COMP is clamped between one diode drop above 3.9V

(approximately 4.6V) and one diode drop below 1.7V

(approximately 1V) to prevent speed loop “wind-up”. Speed

loop compensation components are connected to this pin as

shown in Figure 9. The speed loop compensation compo-

nents are calculated as follows:

Figure 9. Speed Control Loop Component Connections

The voltage on SPEED COMP is compared with a ramp

oscillator to create a PWM duty cycle. The PWM ramp oscil-

lator creates a sawtooth function from 1.7V to 3.9V as shown

in Figure 9. A negative clamp at one diode drop below 1.7V

(approximately 1V) starts the oscillator on power up. The

frequency of the ramp oscillator is set by a capacitor to

cycle from the speed control loop is gated the current limit

one shot that controls the LA, LB, and LC output drivers.

C

SC

26.9

N

V

MOTOR

C

VCO

×

×

×

f

SB

K

e

×

2.5

98.696

τm

2

f

SB

2

×

×

+

-------------------------------------------------------------------------------------------

=

(9a)

R

SC

10

2

π f

SB

C

SC

×

×

---------------------------------------

=

(9b)

–

+

+

–

SPEED SET

SPEED COMP

3.9V

1.7V

10k

Ω

20kHz

PWM ON/OFF

FROM ILIMIT

ONE-SHOT

FROM

SPEED FB

TO

GATING

LOGIC &

OUTPUT

DRIVERS

CT

CT

CSC

RSC

1.7V

VREF

C

T

I

f

PWM

--------------

50

µA

×

2.4V

-----------------------------------

=

(10)